English question, please help?

Kirsten is only half-way there. It's everything that begins with a vowel-SOUND in the alphabet, not just 'vowels' ... So we say "an X" and we say "an F" -- for example -- because these two letter-names begin with the SOUND of a vowel ( ' eks ' and 'eff ' ). See what other ones you can find: "an H" has a truly weird name in English, sounding like 'ay-tch'. So it fits in this same 'vowel-SOUND' category too, because a vowel-SOUND is the first thing there... . Probably there might be more. (But not 'y' which STARTS with a 'w' sound, and 'w' is considered a consonant sound....)

an a, a bee, a cee, a dee, an ef, a gee, an aitch, an i, a jay, a kay, an el, an em, an en, an o, a pee, a cue, an ar, an ess, a tee, a u, a vee, a double-u, an ex, a zee [or a zed]

normally when the noun begins in a vowel.

This is an airplane.
This is a dog.

This is an owl.
This is a sparrow.

A vowel. A, e, i, o, u, and y. Everything else, an "a".